The present invention relates in general to video tape recorders and in particular to time-base correction of audio signals reproduced by video tape recorders.
In moving head video tape recorders, one or more magnetic heads carried by a rotating drum record tracks or segments on magnetic tapes with a high writing speed. Each track or segment represents a portion of the incoming television waveform. For example in the NTSC quadruplex format each track contains 16 television horizontal lines, while in typical helical scan formats each track contains one television field.
In the typical helical scan format the recording head is mounted on a drum. The video tape is wrapped around a part of the drum and is transported past the drum. The drum is rotated with respect to the tape in such an orientation that tracks on the tape that come in contact with the recording head and on which video information is recorded begin near one side of the tape and end near the other side. Such tracks are, however, at small angles to the length of the tape so that the tracks are almost parallel to the length of the tape. For example, in one commonly used format, the tracks are approximately 10 centimeters in length on the video tape which is half-inch wide. In other formats, that is, formats other than the helical scan, the tracks or segments may be at greater angles to the length of the tape. For example, the segments may be almost perpendicular to the length of the tape so that the length of the segments is substantially equal to the width of the tape.
In all moving head video formats, the orientation of the tape with respect to the drum, the position of the recording head on the drum, the transport speed of the tape and the speed of rotation of the drum must be quite precise. Thus, when a tape recorded by a moving head type video tape recorder is reproduced, the position of the playback head on the drum, the relative position of the drum with respect to the tape and the transport and rotation speeds of the tape and drum will affect the quality of the reproduced video information. Since the tracks or segments are recorded successively, they are reproduced also successively. To enable the playback head to accurately track the video segments on the tape, servo mechanisms have been employed. However, such mechanisms do not correct the time distortions of audio signals of the type explained below.
Because of the effects of temperature and humidity, or because a different machine is being used to reproduce the tape, the relative dimensions of the tape and the drum may be different from those applying during the recording. As a result, the information from each track will be slightly compressed or expanded in time compared with the original television waveform. When the reproduced signal is to be displayed on a television screen, this discrepancy between the original duration of each picture field and the reproduced duration does not affect the display, since during the visible part of the picture field the effect is merely a minor change in the horizontal dimension, and the time discontinuity occurs during the invisible field flyback period. In television studios it may be necessary to reproduce a television waveform with regular horizontal synchronizing pulses to permit mixing and cutting between sources. A time-base corrector may then be used to expand or compress each track's information in time to remove the discrepancies; such a procedure involves storage devices with a bandwidth equal to or greater than the television bandwidth, and is therefore only feasible in costly professional equipment.
In addition to the video, it is possible to record audio with the rotating heads using a modulated carrier technique. For example the format known as Betahifi employs a number of frequency-modulated carriers, accommodated in a region of the recorded spectrum between the luminance and the (color-under) chrominance. Both the video and audio information are fed to the same recording head and then recorded on the same track or segment on the video tape. Thus, in the Betahifi format the audio information is differentiated from the video information by being limited to different frequency ranges. In the VHS format, however, the audio and the video information may overlap in frequency. Instead, in the VHS format, two recording heads are used to record the video and audio signals separately. The recording head recording audio information generates a stronger magnetic field than the recording head recording video information so that the audio information resides in the portion of the tape away from the recording heads while the video information resides in a layer of the tape adjacent to the recording heads. In either format, however, the effects of temperature, humidity, different dimensions of tape and drum or a reproducing setting different from the recording setting affect the reproduction of audio information in the same manner as the reproduction of video information.
During reproduction each segment or track containing audio information will be compressed or expanded in time, so that successive segments of audio do not join together correctly; at the moment of switching from one track to the next, there will be a discontinuity in the output audio waveform. This discontinuity, at the beginning of each segment, must not be confused with any disturbance resulting from imperfect switching. In the unlikely event of no time compression or expansion, a switching disturbance may occur, but it is the result of imperfections in the circuitry. The disturbance resulting from time compression or expansion is inherent in the recording and reproduction process, and gives rise to a crackly buzz whose subjective level increases with increasing audio modulating frequency. This flaw in audio reproduction would clearly be eliminated by the application of a wideband time-base corrector which would expand or compress the time-scale for each track so that all the reproduced information, video and audio, is continuous through the track switching. It would also be possible to use time-base correction techniques to correct the timing of the audio carriers only before demodulation, but this would still require wideband circuits. Wideband time-base correctors are complex and expensive and are not feasible except for professional equipment such as those in television studios.